Hi there,
I thought it’s a good idea to document the solution for other users with the same problem.
I do have an MT300n-V2 as well, but I guess it will work for other GL Inet products as well.
I had a chat with the Mullvad support facing the same problem, and they replied fast and with a solution to the problem.
All you have to do is go to Account | Mullvad choose Android as your Platform, ignore the instructions on the right side as they seem to be for Android mobile phone/tablet devices.
Enter your Mullvad Account number, and you can leave the Standard Port selection or pick a Port of your choice and then download the Android configuration which is a single .ovpn file that for the MT300N-V2 is compatible as is.
Just upload it via the Router UI and enter your Mullvad Account Number for username and “m” for the password when you are asked for it. The rest is self-explanatory, pick the options/config you want in the Router UI and press connect.
Since both, GL Inet products and Mullvad seems to be a popular option for many users I suggested Mullvad get in touch with GL Inet so you can add the process to your documentation and list of compatible VPN providers and Mullvad can add some instructions specific to GL Inet products to their website.
Hope I could help.
Thanks for providing a great product.